Choose the correct tens form to complete each sentence
1.He...for 3 years before he entered the university.
a)had studied studying
B) had been studying
C) studied
D) was
2.He understood she...for him for half an bour already
A) had been waiting
B) waited
C) was waiting
D) had waited
3.They... on this problem since morning but nothing was certain.
A) was working worked
B) worked
C) had been working
D) had ​